1. Field of the Invention
The present invention relates to a roller excellent in elasticity and in mold releasing ability, and more particularly to a fixing roller for fixing a toner image by applying at least pressure thereto and a fixing device utilizing such roller.
2. Related Background Art
Although various image fixing devices have been proposed and practiced, there is principally employed, at present, so-called heat roller fixing device consisting of paired rollers having a heat source in at least one of said rollers, as shown in FIG. 3, in consideration of the thermal efficiency. In such fixing device, a heating roller 1 having a heat source (halogen lamp in this case) 3 and coming into contact with the unfixed toner and a pressure roller 2 usually having an elastic layer are maintained in pressure contact with a suitable nip width, and are rotated by drive means (not shown) in directions indicated by arrows. The surface temperature of the heating roller 1 is detected by a thermistor 4 and is maintained at an appropriate value. Upon passing of a recording sheet bearing an unfixed toner image thereon between said paired rollers, the thermofusible toner on said sheet is fixed by heat and pressure. The thermal conduction in such device is significantly better than in other fixing methods, because of direct contact of the toner with the fixing roller equipped with heat source therein.
However, such heat roller fixing method is associated with an offset phenomenon, which is the transfer of toner onto the roller. Said offset phenomenon is caused by a lack of temperature control, including low-temperature offsetting resulting from lowering of the surface temperature of the heating roller and high-temperature offsetting resulting from elevation of said surface temperature, and from an electrostatic factor, resulting from attraction or repulsion between the surface potential of the: roller and the charge of the toner.
Said temperature factor can be eliminated by precise control of the surface temperature of the heating roller, but the electrostatic factor is difficult to eliminate.
In order to reduce the surface potential of the heating roller, it has been proposed to compose the surface layer of the heating roller with a material of low resistance and to ground said surface layer.
The surface of the pressure roller is also required to have excellent releasing ability for the toner and to be flexible under pressure to form a satisfactory nip,
Said pressure roller, maintained in pressure contact with said heating roller, may also be charged up by frictional charging, and may lead to electrostatic offsetting of the toner.
However, if a material of low resistance is contained in the surface layer of the pressure roller in order to preventing the charging thereof, the releasing ability of the surface is deteriorated, so that the surface of the pressure roller is smeared by toner adhesion thereon after fixing of many sheets. In an even worse case, the toner on the roller surface is again offset onto the rear surface of the copy sheet, thus smearing said rear surface.
Such phenomenon is particularly evident in a two side or both-face image forming apparatus in which the pressure roller comes into contact with the toner after in fixing, and a device lacking the cleaning mechanism for the pressure roller.
For this reason there is employed a charge eliminating brush, but such brush cannot provide enough effect for preventing the charge generation at the nip of the rollers, and loses the charge eliminating ability when the brush is smeared with toner or paper dust.